High efficiency flyback converter

ABSTRACT

A DC-DC flyback converter that has a controlled synchronous rectifier in its secondary circuit, which is connected to the secondary winding of a main transformer. A main switch (typically a MOSFET) in the primary circuit of the converter is controlled by a first control signal that switches ON and OFF current to the primary winding of the main transformer. To prevent cross-conduction of the main switch and the synchronous rectifier, the synchronous rectifier is turned ON in dependence upon a signal derived from a secondary winding of the main transformer and is turned OFF in dependence up a signal derived from the first control signal. In one embodiment the first control signal is inverted and delivered to a logic circuit along with the voltage across the main transformer secondary winding and the voltage across the synchronous rectifier. In a further embodiment the first control signal is differentiated and supplied to a control primary winding wound on the outer flux paths of a main transformer core that has a center flux path on which is wound the main transformer primary and secondary windings. A control secondary winding is wound on the outer flux paths in current canceling relation as to flux conducted from the center flux path into the outer flux paths. The control signal for the synchronous rectifier is taken from the output of the control secondary winding in this latter embodiment.

CROSS REFERENCE TO RELATED APPLICATIONS

This is the U.S. national stage of Patent Cooperation Treaty (PCT)application No. PCT/CH03/00245 from which priority is claimed, and whichclaims priority from U.S. provisional patent application Ser. No.60/372,280 entitled “High Efficiency Flyback Converter” filed Apr. 12,2002 in the name of Ionel D. Jitaru. That application is incorporatedherein by reference.

FIELD OF THE INVENTION

This invention relates to DC-DC flyback converters and more particularlyto a DC-DC flyback converter having a controlled synchronous rectifierin a secondary circuit and the means of controlling the synchronousrectifier.

BACKGROUND OF THE INVENTION

Of available DC-DC converters used for power conversion purposes, theflyback converter is the most simple. Its minimum configuration consistsof only a switch, a transformer, a diode and two capacitors (one at theinput port and the second one at the output port). In low-output-voltageconverters, the conduction loss of the diode rectifier due to itsforward voltage drop becomes the dominant power loss. In certainconditions this loss can reach 50% of the total power loss. The simpleapproach conceived to reduce the above-mentioned power loss was toreplace the rectifying diode with a synchronous rectifier, i.e. with alow-ON-resistance MOSFET. To perform the normal operation of the flybackconverter the control of the MOSFET used as the synchronous rectifier iseasily obtained by inverting the primary main switch control signal.FIG. 1 shows such an approach. There an input voltage source 2 suppliesa series-connected primary winding 6 of a transformer 4 and a primaryswitch S1. A control signal Vc(S1) for the switch S1 is constantfrequency control signal having a variable ON duty-cycle to assure astable output voltage. To the output circuit, a secondary winding 8 ofthe transformer 4 provides a voltage 10 that alternates in polarity. Thesynchronous rectifier S2 couples an output load circuit comprising aload 24 and a filtering capacitor 22 to the output voltage 10 during theOFF time of S1's switching period. A synchronous rectifier S2 receives acontrol signal Vc(S2) through an inverter 26. Because of its effectduring operation of the circuit, the body diode 18 of the MOSFET that isused as a synchronous rectifier is also shown in FIG. 1.

FIG. 1A displays the voltages and currents versus time waveforms for theflyback circuit parameters, i.e., Vc(S1), the control signal for theswitch S1; Vc(S2), the control signal for switch S2 or synchronousrectifier; I(S1), the current through the switch S1; I(S2), the currentthrough the switch S2 and V(S2), the voltage across the switch S2, forcontinuous-conduction mode (CCM). FIG. 1B displays the voltages andcurrents versus time waveforms for the discontinuous-conduction mode ofoperation (DCM). FIG. 1C depicts the same waveforms for the criticalconduction mode of operation, the limit case between continuous mode anddiscontinuous mode of operation.

The main drawback of the above presented approach is thecross-conduction (concurrent conduction) of the primary switch S1 andsecondary synchronous rectifier S2 for the time intervals t₀-t₁, t₂-t₃(see FIG. 1A), for CCM operation. As a result, during these timeintervals when the body-diode 18, an intrinsic part of the synchronousrectifier S2, is ON, supplementary power losses appear and have to betaken into account. Another drawback is related to power loss introducedby the reverse recovery, during t₃-t₄, of the body-diode 18 duringturn-off of the primary switch S1, which adds to the general powerbalance or total power use of the circuit.

For DCM operation, See FIG. 1B, the power loss due to reverse recoveryprocess of the body-diode 18 is eliminated because the current of thesynchronous rectifier S2 becomes zero before the primary switch S1 isturned “on,” but the cross-conduction process during turn-off of switchS1 still exists. There are two drawbacks related with this mode ofoperation: higher conduction loss during Td (ON), see FIG. 1C, due tocross-conduction of I(S1) and I(S2) and the fact that the circuitefficiency fluctuates with Vin of the input voltage source 2.

The critical conduction mode of operation offers in certain conditions azero voltage switching or ZVS feature which can be used if a properdesign of the circuit is made. In this case the recover loss iseliminated also but the higher conduction loss associated withcross-conduction of I(S1) and I(S2), during Td(ON) delay, is stillimportant.

There is a need therefore for a DC-DC flyback converter having asynchronous rectifier in its secondary circuit that is controlled suchthat cross-conduction losses are eliminated or substantially eliminatedand reverse recovery losses are eliminated or substantially eliminated.

SUMMARY OF THE INVENTION

In order to address the drawbacks of the prior art, the presentinvention provides an improved driving circuit that reduces power lossdue to cross-conduction process.

In accordance with a preferred embodiment of the present invention, adriving technique is provided which improves the efficiency of a flybackDC-DC converter using a synchronous rectifier in the output section orsecondary circuit by processing in a certain sequence informationrelated to the primary switch control, the voltage across the secondarywinding of the power transformer and the voltage across the synchronousrectifier to obtain a final control signal for the synchronousrectifier.

In one embodiment of the present invention, the control circuit for asynchronous rectifier in the secondary circuit comprises a logic circuitthat uses the control signal that is used to control the main switch orprimary switch in the primary circuit and the voltage derived from asecondary winding of a main transformer coupling the primary andsecondary circuits. The logic circuit also uses the voltage across thesynchronous rectifier to form a third input to the logic circuit. Thesignals developed turn ON and turn OFF the synchronous rectifier. Morespecifically, the logic circuit of the embodiment of the invention justdescribed includes an AND gate to which a voltage derived (inverted)from the main switch control signal is applied along with outputs fromfirst and second bistable circuits. The first bistable circuit issupplied an input from a first comparator to which the voltage derivedfrom a main transformer secondary winding is applied. The secondbistable circuit has an input from a second comparator to which issupplied the voltage across the synchronous rectifier. To the first andsecond bistable circuits the main switch control voltage is applied as asecond input. In this way the main switch control signal turns OFF thesynchronous rectifier and the voltage across the secondary winding turnsON the synchronous rectifier. Reference inputs to the comparatorstypically are set at substantially zero volts. In the preferredembodiment that uses the logic circuit to develop the synchronousrectifier control, typically the first and second bistable circuits, arefirst and second RS flip-flops. The voltage derived from the firstcontrol signal is applied to the “set” or S input of each of theflip-flops. To the “reset” or R inputs are applied the outputs of thefirst and second comparators, respectively. The first flip-flop has its“set” or Q output applied as an input to the AND gate and the secondflip-flop has its “reset” or Q output applied as an input to the ANDgate.

The method of controlling induction of the synchronous rectifier in thesecondary circuit of a DC-DC flyback converter, then, preferablyincludes turning on the synchronous rectifier in dependence onestablishment of a voltage across a transformer secondary winding of themain transformer that couples the secondary circuit to the primarycircuit and turning OFF the synchronous rectifier in dependence onturning ON the main switch in the primary circuit. Turning OFF thesynchronous rectifier in dependence or the turning ON of the main switchpreferably comprises turning OFF the synchronous rectifier when the mainswitch is turned “on.” As previously described the method can includeproviding a logic circuit connected with the control electrode of thesynchronous rectifier for the application of the voltage derived from avoltage across a secondary winding as one input to the logic circuit andapplying a voltage derived from a main switch control signal as afurther input to the logic circuit. In a preferred embodiment the thirdinput to the logic circuit is a voltage derived from a voltage acrossthe synchronous rectifier.

The method of a specific, exemplary preferred embodiment can beutilization of the logic circuit as described in greater detail above inconnection with the use of an AND gate, a pair of bistable circuits, andsupplying the main switch control signal to an inverter connected withthe AND gate. In this method, the provision of the bistable circuits canbe the provision of first and second RS flip-flops as previouslydescribed. Also as previously described the method can include theapplication of the specific inputs to the set and reset inputs of theflip-flops and the specific set and reset outputs applied to the ANDgate.

In a further embodiment of the invention the secondary winding fromwhich the control signal for the synchronous rectifier is in partderived can be a control secondary winding wound on a magnetic core thatserves as the main transformer core, the magnetic core having a centerflux path dividing into two outer flux paths onto which the secondarywinding is wound in current canceling relation as to flux that isconducted from the center flux path to the two outer flux paths. Aprimary control winding on at least one of the two outer flux paths issupplied an input derived from the main switch control voltage.

In a preferred embodiment the main switch control voltage isdifferentiated and the differentiated signal is applied to the controlprimary winding input. In this preferred embodiment, the main switchcontrol signal is substantially a square wave.

In one specific exemplary preferred embodiment, the control voltagedeveloped in the control secondary winding of the embodiment of theinvention just described is applied to a switching circuit connected incontrolling relation to the synchronous rectifier. The switching circuitmay be a transistor switching circuit. In the case of a transistorswitching circuit, a DC bias voltage can be developed from a secondarywinding of the main transformer and applied to the transistor switchingcircuit for the purpose of applying a DC bias thereto. In each of thepreferred exemplary embodiments described, the synchronous rectifier canbe a MOSFET switch. Where a transistor switching circuit is connected tocontrol the synchronous rectifier, it can be connected to the gate ofthe MOSFET switch. One preferred embodiment of a suitable transistorswitching circuit is a serially connected PNP and NPN transistor pairconnected between the DC bias voltage and ground, the junction of thetransistor pair being connected to the gate of the MOSFET switch.

A preferred embodiment of a DC-DC flyback converter in accordance withthe invention is a flyback converter having, as previously described, asynchronous rectifier in its output or secondary circuit to which issupplied a control signal derived from a voltage across a controlsecondary winding that is wound upon two outer flux paths of a magneticcore serving as the magnetic core of the main transformer and arrangedsuch that flux conducted from a center flux path into the outer fluxpaths develops substantially no current in the control secondarywinding. A control primary winding is wound onto at least one of the twoouter flux paths and preferably, in one preferred embodiment, onto eachof the two outer flux paths in current canceling relation with respectto the flux conducted to the outer flux path from the center flux path.An input to a circuit supplying the control primary winding with aninput taken from the primary or main switch control signal is,preferably, a differentiation circuit connected between the control ofthe main switch and the control primary winding. In this flybackconverter, the control signal applied to the synchronous rectifier canbe supplied by the switching circuit described above. Again, theswitching circuit can be a transistor switching circuit and can have abias potential applied thereto taken from a DC bias circuit connectedwith a secondary winding and including at least one rectifying diode.The turning ON and OFF of the transistors that make up the transistorswitching circuit is, of course, taken from the control secondarywinding.

In the preferred exemplary embodiments described herein, power lossesdue to cross conduction between the main switch and the synchronousrectifier can be eliminated or nearly entirely eliminated and reverserecovery losses can likewise be eliminated or nearly entirelyeliminated.

Both of the above-described embodiments of the present can beimplemented by using either a separate transformer or one integratedwith the main transformer. The transformer can be discrete component ofits own or can be imbedded in a PCB carrying the converter circuitry.The driving transformer in both embodiments can be embedded into themain transformer of the converter.

The foregoing and other objects, features and advantages of the presentinvention will be more readily understood upon consideration of thefollowing detailed description of the invention together with thefollowing drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a prior art flyback converter with asynchronous rectifier in the output section;

FIG. 1A is a series of plots of the main voltage and current waveforms,versus time, for the continuous-conduction mode of operation or CCM ofthe prior art converter of FIG. 1;

FIG. 1B is a series of plots of the main voltage and current waveforms,versus time, for the discontinuous-conduction mode of operation or DCMof the prior art converter of FIG. 1;

FIG. 1C is a series of plots of the main voltage and current waveforms,versus time, for the critical-conduction mode of operation, the limitcase between CCM and DCM, of the prior art converter of FIG. 1;

FIG. 2 illustrates the basic schematic of the flyback converter withsynchronous rectifier in the output section according to this invention;

FIG. 3 is a schematic of a synchronous rectifier like that of theconverter of FIG. 2 including the control according to this invention;

FIG. 4A is a series of plots of the main voltage and current waveforms,versus time, for the continuous-conduction mode of operation or CCM ofthe converter of FIG. 2 according to this invention;

FIG. 4B is a series of plots of the main voltage and current waveforms,versus time, for the discontinuous-conduction mode of operation or DCMof the converter of FIG. 2 according to this invention; and

FIG. 5 is a series of plots of the driving technique of the synchronousrectifier of the converter of FIG. 2 according to this invention.

DETAILED DESCRIPTION

The approach used to control the synchronous rectifier of a DC-DCflyback converter according to this invention is first described inrelation to the improved basic schematic of the flyback converter shownin FIG. 2. In the flyback converters of FIGS. 1 and 2, like elementsbear like reference numerals. The voltage source 2 supplies the inputcircuit formed by the primary winding 6 of the power transformer 4connected in series with the switching MOSFET S1. The switchingtransistor S1 is controlled by a signal Vc(S1). The output circuit ofthe flyback converter contains the secondary winding 8 of the powertransformer 4 connected in series with the synchronous rectifier S2 andthe output load 24. The output voltage obtained on the load is filteredby the capacitor 22. The body diode 18 of the synchronous rectifier isalso represented as it plays a role in the operation of the circuit.According to one preferred embodiment of this invention, besides themain switch control signal Vc(S1), which controls the main inputswitching transistor S2 in FIG. 1, two supplementary signals are used toprocess the final controlling signal Vc(S2) for the output synchronousrectifier S2. These are the voltage 50, Vs(+)-Vs(−), across thesecondary winding and the voltage Va-b across the synchronous rectifierS2. This is described with reference to FIG. 3.

In FIG. 3 a control section 100 is used to process digitally the signalVc(s2). The main switch control signal Vc(S1) is first coupled throughan inverter 101 to the multiple input AND gate 116. Also the signalVc(S1) is coupled to the “set” (S) inputs of two bistable circuits 112and 114. These may be RS flip-flops as shown. The voltage 50 across thesecondary winding 8, Vs(+)-Vs(−), is coupled to the non-inverting input(+) of a comparator circuit 102. On the inverting input (−) is coupled areference signal Vref1, which typically is equal to zero, but could havea non-zero value also. The output signal of the comparator circuit 106is connected to the “reset” (R) input of the bistable circuit 114. Theoutput “set” (Q) state of the bistable circuit 112 and the output“reset” (Q) state of the bistable 114 are each connected to one of themultiple inputs of the AND gate 116. The output of the AND gate 116provides the control signal Vc(S2) for the synchronous rectifier S2. Themain features this circuit assures are: the turn ON of the synchronousrectifier S2 is provided by the voltage 50 on the secondary winding,Vs(+)-Vs(−), and turn OFF is by the information coming from the primaryside, Vc(S1). Cross-conduction of the main switch S2 and the synchronousrectifier S2 is avoided.

FIG. 4A depicts the main voltage and current waveforms versus time forthe converter of FIG. 2 operating in continuous-conduction mode and withthe improved control of the digital logic circuit depicted in FIG. 3.FIG. 4A shows the drain-source voltage Vds(S1) on the main primaryswitch S1. Also shown is the voltage on the secondary winding Vsec(Vs(+)-Vs(−)), which takes into account the delay produced in thetransformer by the leakage inductance and the level of the currentthrough it. FIG. 4A shows, as well, the effect on the control signalVc(S2) produced by a heavy or a light load. I(S2), the current throughthe synchronous rectifier is also shown in FIG. 4A. Shown in brokenlines is the reverse current 30 eliminated by the “adjustable” turn ONby Vc(S2) of the synchronous rectifier to eliminate reverse recoverylosses.

FIG. 4B depicts the main voltage and current waveforms versus time forthe converter of FIG. 2 operating in discontinuous-conduction mode, andwith the improved control of the digital logic circuit depicted in FIG.3. FIG. 4B shows the drain-source voltage Vds(S1) on the main primaryswitch S1. Also shown is the voltage on the secondary winding, Vsec(Vs(+)-Vs(−)), which again takes into account the delay produced in thetransformer by the leakage inductance and the level of the currentthrough it. FIG. 4B shows, as well, the voltage Vds(S2) across thesynchronous rectifier S2 and the effect on the control signal Vc(S2)produced by a reduced load.

Another embodiment of this invention is exemplified by the drivingtechnique used to control the synchronous rectifier S2 as depicted inFIG. 5. The driving technique can make use of a transformer separatefrom the main transformer, or transformer windings integrated with themain transformer as illustrated in FIG. 1. Either way, the transformercan be a discrete transformer or a planar transformer imbedded in a PCB.An input signal source 60 is applied through a derivative ordifferentiation (dv/dt) RC circuit 62, 64 to a control primary winding66 wound on the outer flux paths 70, 74 of an E+I magnetic core assembly76, 80. The input signal 60 is ordinarily the control signal for themain switch S1 connected in the input, primary circuit (as shown in FIG.1). FIG. 5A plots, at 61, the voltage of source 60 at point A in FIG. 5,and its derivative, at 63, provided at point B in FIG. 5 vs. time. Themain windings 78 of the power transformer are wound on the center leg72. Control secondary winding 68 is also wound on the outer flux paths70, 74. The control secondary winding 68 is wound on the outer fluxpaths such that flux passing into the outer flux paths from the centerflux path of the leg 72 results in substantially no current in thewinding 68. In the embodiment of FIG. 5, the control primary winding 66is similarly wound. Flux produced in the outer flux paths 70, 74 cancelin the center leg 72. In this way the windings 66, 68 of the drivingtransformer produce a magnetic field that doesn't interfere with themain one produced by the power transformer as long as the driving fieldis not too high, and the control secondary winding current is the resultof substantially only the control primary winding excitation. The outputsignal produced by the secondary winding 68 is applied to a totem-polestructure made by serially connected NPN transistor 84 and PNPtransistor 82, which structure delivers the control signal to thesynchronous rectifier 86. The totem-pole structure is supplied with abiasing DC voltage produced by an auxiliary winding 90. This ispreferably also wound on the main transformer and is rectified with arectifying diode 88. The operating characteristics of this embodimentare essentially the same as those of the converter operated undercontrol of the circuit of FIG. 3, as plotted in FIGS. 4A and 4B.

A main advantage of this embodiment as shown is the simplicity of thedriving circuit. At the same time the integrated option offers the lowercost which can be obtained because the synchronous rectifier 66 and 68driving windings are designed and made together with main windings 78 ofthe power transformer. Note that the “outer flux paths” on which thewindings 66 and 68 are wound include not just the outer legs 70 and 74but the entire magnetic structure forming the two paths out of and thenreturning to the center leg. In other words either or both windings 66and 68 can be wound on the upper or lower paths adjoining the center legas shown in FIG. 5 as well as on the two outermost legs of the magneticcore 76, 80.

The foregoing descriptions of preferred embodiments are exemplary andnot intended to limit the invention claimed. Obvious modifications thatdo not depart from the spirit and scope of the invention as claimed willbe apparent to those skilled in the art. For example, although the logiccircuit of FIG. 3 uses specific bistable circuits and gate, other logicelement may be arranged to accomplish the same or a similar output fromthe inputs applied. And in the embodiment of FIG. 5 “E” and “I” magneticcore elements are employed, but a toroid bridged by a central leg couldprovide the outer flux paths and the center leg for example. Switchingcircuits other than the series connected transistors 82 and 84 may beeffectively employed as well.

1. A method of controlling conduction of a synchronous rectifier in asecondary circuit of a DC-DC flyback converter comprising: (a) turningON the synchronous rectifier in dependence on the establishment of avoltage across a secondary winding of a main transformer inductivelycoupling the secondary circuit to a primary circuit of the flybackconverter, and (b) turning OFF the synchronous rectifier in dependenceon turning ON of a main switch in the primary circuit in currentcontrolling relation to a primary winding of the main transformer. 2.The method according to claim 1, wherein step (b) comprises turning OFFthe synchronous rectifier in dependence on a control signal turning ONthe main switch.
 3. The method according to claim 1, wherein thesecondary winding is a control secondary winding, steps (a) comprisingproviding in the main transformer a magnetic core having a center fluxpath and two outer flux paths, winding the control secondary winding onthe two outer flux paths in current canceling relation as to fluxconducted to the two outer flux paths from the center flux path, windinga control primary winding on at least one of the two outer flux paths,winding on the center flux path at least a main primary winding inenergy communicating relation to a main secondary winding.
 4. The methodaccording to claim 3, further comprising winding the main secondarywinding on the center flux path.
 5. The method according to claim 3,wherein winding a control primary winding comprises winding the controlprimary winding on the two outer flux paths in flux canceling relationwith respect to the center flux path.
 6. The method according to claim3, further comprising applying a control voltage developed in thecontrol secondary winding to control the switching ON and OFF of thesynchronous rectifier.
 7. The method according to claim 3, furthercomprising applying to the control primary winding a differential signaldeveloped by differentiation of a main switch control signal.
 8. A DC-DCflyback converter having a main transformer, a DC input connection in aprimary circuit connected with a primary winding of the maintransformer, a controllable primary switch in series with the primarywinding of the main transformer, said controllable primary switch havinga primary control signal input connection for applying a first controlsignal in controlling relation to the controllable primary switch, aload connection in a secondary circuit connected with a secondarywinding of the main transformer, a synchronous rectifier connected inseries with the secondary winding and having a control connection forapplying a second control signal to the synchronous rectifier incontrolling relation to the synchronous rectifier, a control circuitcoupled between the primary control signal input connection and thecontrol connection for applying the second control signal to thesynchronous rectifier control connection, the improvement comprising:the control circuit comprising a control primary winding and a controlsecondary winding wound on the main transformer, the control primarybeing connected in a circuit having an input from the primary controlsignal input connection, the control secondary winding being connectedin controlling relation to the control connection of the synchronousrectifier, the main transformer having a magnetic core with a centerflux path on which is wound a main primary winding, and the magneticcore having two outer flux paths on both of which is wound the controlsecondary winding, the control primary winding being wound on at leastone of the outer flux paths, the control secondary winding being woundon the two outer flux paths in current canceling relation with respectto flux conducted to the two outer flux paths from the center flux path,whereby a control signal generated in the control secondary winding issubstantially unaffected by flux developed in the main transformer bycurrents in the main primary winding.
 9. The DC-DC flyback converter ofclaim 8, wherein the control primary winding is wound on both of the twoouter flux paths in flux canceling relation with respect to the centerflux path.
 10. A DC-DC flyback converter having a main transformer, aprimary circuit including a controllable switch connected in currentcontrolling relation to a primary winding and having a controlconnection for opening and closing the switch under the control of afirst control signal, a secondary circuit for supplying an output to aload, and a synchronous rectifier having a control connection foropening and closing the synchronous rectifier, the improvementcomprising: a control circuit coupled between the control connections ofthe controllable switch and the synchronous rectifier comprising (a)means for turning ON the synchronous rectifier in dependence on avoltage developed across a secondary winding on the main transformer,and (b) means for turning OFF the synchronous rectifier in dependence onthe first control signal turning ON the controllable switch.